Embodiments provide methods, devices, circuitry and systems for supporting or implementing functionality to provide a filtering circuit, and for providing a transmitter, receiver or transceiver incorporating the same. One embodiment provides a circuit for filtering signals. The circuit outputs a plurality of signals at a plurality of frequencies, each of the signals being attenuated by an amount determined by the frequency of the signal. The circuit attenuates signals output at a first frequency by an amount which is low relative to the attenuation of signals output at frequencies other than the first frequency. The circuit further attenuates signals output at a second frequency by an amount which is high relative to the attenuation of signals output at frequencies other than the second frequency. Responsive to a change in the control signal, the circuit changes the first and second frequencies while maintaining a fixed ratio between the first and second frequencies.